"Now President Obama has joined that silence," the two write. "Mr. Obama has managed to avoid scrutiny about his most tragic foreign-policy failure: standing by as Sudan's Islamic regime perpetrates a slaughter against its own citizens who belong to non-Arab ethnic groups."

Bashir has killed tens of thousands of his own people, the pair note, adding that Obama, meanwhile, cites "humanitarian reasons to intervene in a crisis" only "when politically convenient."

"He entered Libya 'to prevent a bloodbath,' despite no mass slaughter of civilians in that country. This disingenuous explanation only damaged his credibility as a humanitarian, though it did placate the international human-rights community," Farrow and Goldhagen say, adding that Sudan merits at least the same treatment.

"An enormous number of civilian lives are at risk in Darfur. Bashir's assaults against the people of Sudan have escalated in intensity. Some three million people are living — if you can even call it that — in refugee camps under wretched conditions."

They say the outside world, meanwhile, doesn't know what's going on in Sudan because it's not letting in outsiders, including the media.

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"There is a moral imperative to help the people of Darfur, which President Obama once articulated well," Farrow and Goldhagen continue, referencing a statement he made in 2007 calling the "slaughter of innocents" wrong and advocating a "protective force on the ground."

But, "those suffering in Darfur can expect no such 'protective force' from the U.S.," they write.

"For reasons that are unclear, Sudan doesn't meet Mr. Obama's threshold for action . . . Mr. Obama has abandoned his own moral standards and left the people of Darfur to perish."